<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    .flex-box {
      display: flex;
      height: 400px;
      width: 100%;
      background-color: red;
    }
    
    .left {
      width: 200px;
      background-color: yellow;
    }

    .right {
      width: 300px;
      background: #000;
    }

    .center {
      flex: 1;
      background-color: blue;
    }
  </style>
</head>
<body>
  <div class="flex-box">
    <div class="item left">left</div>
    <div class="item center">center</div>
    <div class="item right">right</div>
  </div>
</body>
</html>